1. Field of the Invention
The present invention relates to a stylus, in particular, to a stylus for a coordinate-measuring machine and having a stem with a break-off point. The present invention also relates to a method of forming a stylus with a break-off point, and to a coordinate-measuring machine having a stylus with a break-off point.
2. Description of the Prior Art
As known, styluses are used in coordinate-measuring machines which perform different measuring tasks from a simple measurement of a distance between two spaced from each other objects to a measurement of the three-dimensional surfaces.
A stylus, which is usually used in coordinate-measuring machines, usually has a cylindrical stem formed of steel or ceramics and at one end of which, a mechanical touch element is attached, mostly, in a form of a ground ruby ball having a predetermined radius. The touch element is used for taking measurements of a specimen. The other end of the stem is connected with the touch probe which, on one hand, provides for displacement of the stylus and, on the other hand, contain sensors for determining the stylus displacements.
Coordinate-measuring machines can be divided in different groups. The first group includes linear measuring systems which output a position of a stylus, which is displaced in a measuring direction, as a measurement value.
The linear measurement systems permit to conduct simple distance measurements or obtain a height profile of a workpiece movable relative to the stylus.
The second group includes coordinate-measuring machines having a resiliently biased stylus that can be deflected in any direction. These so-called three-dimensional measurement systems function based on different principles.
Three-dimensional systems can include linear and rotary encoders which completely determine the deviation of the stylus and, thereby, provide for determination of coordinates of the touch element. These machines are designated as measuring touch systems. They insure determination of measurement values with a statical or dynamic operation, i.e., specimen and the touch element can, at the time of the acquisition of a measurement value, remain stationary with respect to each other or displace relative to each other.
Also known are so-called switching touch systems. With these systems, a trigger signal is generated at a predetermined deviation of the stylus. The signal is transmitted to a control unit which then receives coordinates from an available measuring system, e.g., of a machine tool. The switching systems operate only dynamically. After generation of the measurement signal, the displacement of the specimen or the touch system should be stopped in order that the maximum allowable deviation of the stylus is not exceeded.
It is not a trivial object to determine, in all cases, based on a position of the stylus, with a touch probe, the coordinates of the measurement point, i.e., the contact point of the touch element with the specimen. For the determination of coordinates, the measurement direction, deflection of the stylus, geometry of the touch element, and direction of the surface perpendicular of the specimen in the measurement point, all need be considered. The discussion of the coordinate determination can be found in an article of T. Pfeifer and A vom Hemdt “Berechung der Basiselemente und die Tasterkompensation in der koordinatenmesstechnik (Calculation of Basic Elements and Stylus Compensation)” published in a magazine “Technisches Messen (Technical Measurements)” 5/90, published by R. Oldenbourg Verlag.
Flexural characteristics, i.e., flexural strength of the stylus, also play an important role in the quality of the measurement with a coordinate-measuring machine.
In order to prevent damage of the sensitive mechanics or of the measuring system in the touch probe in case of an uncontrolled collision of the stylus with the specimen or other parts, it is desirable to provide the stylus with a predetermined break-off region that would prevent transmission of a large force.
German Publication DE 33 14 318 discloses a coordinate-measuring machine including an inductive measurement system that can determine a position of a stylus in one direction. The predetermined break-off region is obtained by a noticeable reduction of the cross-section of the stylus stem. The predetermined break-off region should protect this unidimensional measuring system against a large load acting transverse to the measurement direction. In this case, a particularly high flexural strength of the stylus is not required as bending of the stylus in a direction transverse to the measuring direction only slightly influences the measurement.
An object of the present invention is to provide a stylus having a predetermined break-off region and which can be economically produced and the flexural strength of which is not reduced at all or is reduced only insignificantly by the break-off region.
Another object of the present invention is to provide a cost-effective method of producing a stylus having a predetermined break-off region and which would not affect or affect only insignificantly the flexural strength of the stylus.
A further object of the present invention is to provide a coordinate-measuring machine having a stylus with a break-off region and which can be economically produced and the flexural strength of which is not affected or is affected only insignificantly by the manufacturing process of forming the break-off region.